Some major sellers on eBay use a third party like Marketworks for invoicing, shipping and tracking purposes. Keep an eye on this, because things can go pear-shaped if you use a different method to those requested by the seller.

One of the reasons for using companies like Marketworks, is the seller could be acting as a selling agent who doesn't carry any stocks. The stocks are held and shipped by a 'mother' company. With Marketworks, you should have a tracking number and web link provided and supplied by a response email.